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Voltage Drop Calculator: NEC 3% Wire Gauge Sizer

Calculate voltage drop and conductor percentage loss for copper and aluminum wires based on National Electrical Code (NEC) standards.

Quick Answer:For single-phase circuits, Voltage Drop = (2 × K × I × L) / Circular Mils, where K is conductor resistivity (12.9 for Copper, 21.2 for Aluminum at 75°C).

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How to Use the Voltage Drop Calculator (NEC Wire Sizing)

1Select your circuit supply voltage (120V, 208V, 240V, 277V, or 480V).
2Enter the circuit load current in Amperes.
3Enter the one-way distance between the electrical panel and load in feet.
4Choose wire gauge (AWG/kcmil), conductor metal (copper or aluminum), and phase type.
5Check the resulting voltage drop and verify compliance with the NEC 3% rule.

Formula & How It Works

NEC Chapter 9 Table 8 Circular Mil Formula

1φ: VD = (2 × K × I × L) / CM | 3φ: VD = (√3 × K × I × L) / CM

Uses standard K constants (12.9 for Cu, 21.2 for Al at 75°C) and exact circular mil cross-sections from NEC Chapter 9.

Worked Examples

Standard 120V Outdoor Circuit (20A, 100ft, 12 AWG Copper)

Input: 120V, 20A, 100ft, Copper, 12 AWG (6530 CM), 1-Phase

Explanation: VD = (2 × 12.9 × 20 × 100) / 6530 = 7.90V. Since 6.59% > 3.0%, you should upsize to 10 AWG or 8 AWG.

Result: 7.90V Drop (6.59% drop) — Exceeds NEC 3%

Upsized 10 AWG Copper Run

Input: 120V, 20A, 100ft, Copper, 10 AWG (10380 CM)

Explanation: Significant drop reduction; upsizing to 8 AWG drops it to 3.12V (2.60%), meeting NEC guidelines.

Result: 4.97V Drop (4.14% drop)

Commercial 3-Phase 208V Feeder (50A, 150ft, 4 AWG Copper)

Input: 208V, 50A, 150ft, Copper, 4 AWG (41740 CM), 3-Phase

Explanation: VD = (√3 × 12.9 × 50 × 150) / 41740 = 4.01V. 1.93% is well within the 3% limit.

Result: 4.01V Drop (1.93% drop) — Complies with NEC

Frequently Asked Questions

What is the NEC limit for voltage drop?▼

The National Electrical Code (NEC Informational Note 210.19(A)) recommends that voltage drop not exceed 3% on branch circuits, and not exceed 5% total across feeder and branch circuits combined.

Why does voltage drop matter?▼

Excessive voltage drop causes electric motors to overheat, LED lights to flicker or dim, sensitive electronics to reboot, and wires to waste power as heat.

How do you fix high voltage drop?▼

The simplest solution is to upsize the conductor wire gauge to a thicker wire with a higher circular mil cross-sectional area.

Why is the constant 2 in single-phase and √3 in three-phase?▼

Single-phase current travels out on the hot wire and returns on the neutral (2 conductors). In balanced 3-phase circuits, phase vector cancellation yields √3 (1.732).

Is this calculation private?▼

Yes, all electrical calculations execute 100% locally in your web browser.

Quick Reference & Conversion Table

Wire Gauge	Circular Mils (CM)	Copper Ampacity (75°C)	Aluminum Ampacity (75°C)
14 AWG	4,110 CM	15 A	N/A
12 AWG	6,530 CM	20 A	15 A
10 AWG	10,380 CM	30 A	25 A
8 AWG	16,510 CM	50 A	40 A
6 AWG	26,240 CM	65 A	50 A
4 AWG	41,740 CM	85 A	65 A

⚠️ Common Pitfalls to Avoid

•Entering two-way loop distance instead of one-way distance (the formula's multiplier of 2 already accounts for the return neutral).
•Using room temperature resistivity instead of the standard NEC 75°C operating temperature value.
•Neglecting voltage drop on long outdoor trench runs to sheds, sub-panels, or electric vehicle (EV) chargers.

💡 Practical Real-World Uses

•Sizing underground wire runs to detached garages, workshops, and pole barns.
•Engineering residential EV charger branch circuits (Level 2 240V 48A).
•Commercial HVAC and industrial motor installation planning.

Copper vs Aluminum Conductors

✓Copper has lower electrical resistance (K=12.9), allowing smaller gauge wires for equivalent ampacity.
✓Aluminum has higher resistance (K=21.2) requiring roughly one to two wire gauge sizes larger, but offers major cost and weight savings for large service entrance cables.

Last reviewed & updated: 2026-11-01

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